how do i add mp4 files to edit?
You need to open the AVCHD file as if it were a folder (it is). In Mac: Right click, then "Show contents". You will see another folder, do the same with it. Then you'll find the *.MTS files. Those you can open and work with in Blender.

You can even change the extension "mts" to "mpg", but that's not necessary in Blender.